Various processes have been described in the literature for producing hydrogen cyanide (hereafter HCN) from ammonia (hereafter NH.sub.3) and carbon monoxide (hereafter CO) by passing these compounds in contact with various catalyst materials, including alumina, mixtures of silica gels and alumina, and mixtures of alumina, silica and various other metal oxides. Thus, processes of producing HCN from NH.sub.3 and CO using such types of materials are described, respectively, in U.S. Pat. No. 1,920,795, granted to Alphonse O. Jaeger, Aug. 1, 1933 (hereafter Jaeger patent) and in an article in Zeitschrift fur Elektrochemie 36 1003-7 (1930) by Bredig, E. Elod and Rudolf Muller entitled "Catalytic Formation of Hydrocyanic Acid, III. Formation of Hydrocyanic Acid from Carbon Monoxide and Ammonia" (hereafter Bredig et al article).
Referring now more specifically to the above-identified prior literature disclosures, the Jaeger patent describes a process in which CO and NH.sub.3 at temperatures of 400.degree. to 700.degree. C. and atmospheric or superatmospheric pressure are passed into contact with multi-component zeolites composed of silica, alumina and various metal acids such as vanadyl, chromium, thorium, tungstic acid and molybdic acid and the like. The latter are presumably converted to oxides on calcination. The Jaeger patent teaches that in the formation of such multi-component zeolites by such methods the final reaction product must be alkaline to litmus; and for products of high base exchanging power the zeolites should be neutral or alkaline to phenolphthalein. On the other hand, the Bredig et al article discloses a process in which CO and NH.sub.3 at a temperature of 700.degree. C. are passed into contact with a catalyst which may be a silica gel, a silica-thorium-aluminum oxide gel, a silica alumina gel or an alumina oxide on clay shards. The article states "it was first established that silica gel alone and silica gels mixed with thoria and alumina are considerably inferior in their effectiveness to the other catalysts, including also aluminum oxide on clay shards". The other catalysts referred to in this article appear to be mixtures of alumina and thorium oxide, mixtures of alumina and cerium oxide, or cerium oxide per se.
A study of the Jaeger patent and Bredig et al article indicates that neither experimenter employed an alumina-silica co-catalyst which was highly selective for converting CO and NH.sub.3 to HCN while substantially avoiding the conversion of CO to CO.sub.2 via the water gas shift reaction. This is apparently due to the fact that these experimenters did not recognize what properties were necessary to avoid the conversion of CO to CO.sub.2 via the water shift reaction nor did they disclose processes of preparing catalysts having such properties.